Electrothermographic duplicating sheet

ABSTRACT

AN ELECTROTHERMOGRAPHIC DUPLICATING SHEET IS DISCLOSED. A LAYER COMPRISING A MIXTURE OF AN ELECTROSTATICALLY CHARGEABLE RESIN AND AN ORGANIC HEAT-ACTIVATABLE TACKIFYING AGENT IS ADHERED TO A CONDUCTIVE BASE SHEET. WHEN SUBJECTED TO RADIANT THERMOGRAPHIC HEAT, BOTH A LATENT TACKY IMAGE AND LATENT ELECTROSTATIC IMAGE IS FORMED.

United States Patent 3,672,981 ELECTROTHERMOSGEEAETI-HC DUPLICATING Donald D. Sloan, Weston, and Snresh D. Amberkar,

Framingham, Mass., assignors to Dennison Manufacturing Company, Framingham, Mass. No Drawing. Filed Oct. 2, 1969, Ser. No. 863,365 Int. Cl. B44d 1/18 US. Cl. 117201 Claims ABSTRACT OF THE DISCLOSURE An electrothermographic duplicating sheet is disclosed. A layer comprising a mixture of an electrostatically chargeable resin and an organic heat-activatable tackifying agent is adhered to a conductive base sheet. When subjected to radiant thermographic heat, both a latent tacky image and latent electrostatic image is formed.

SUMMARY OF THE INVENTION Electrophotographic reproduction, i.e. xerography, has been widely used in recent years. This technique involves applying an electrostatic charge to an electrostatically chargeable, photoconductive layer of material containing photo-semiconductors, which are sensitive to light to reduce the electrical resistance of the layer (increase its electrical conductance) and thereby cause the charge to leak away at the light struck areas. The charged layer is exposed to light under a pattern to form a latent electrostatic image of such pattern, which is converted to a visible image by the application of colored toner particles, which may or may not be triboelectrically charged (i.e. they may or may not be electrotropic) and which selectively adhere either to the latent electrostatic image area (positive image) or the latent electrostatic background area (negative image), followed by (l) fixing of the visible image as by heat fusion of the toner particles to the layer or by solvent vapors or (2) transferring the visible image by transfer of the toner particles to another sheet and fixing it on the transfer sheet. The photoconductive layer may be in the form of a selenium drum which, when toner developed, functions as printing plate .to transfer the visivle toner image to a transfer sheet, i.e. paper, or the layer may be in the form of a photoconductive coating over a substrate sheet such as paper.

Over the past ten years in order to overcome certain disadvantages in xerography, including the relatively high cost thereof particularly when a large number of duplicates of a single master are required, there have been a number of proposals and attempts to modify this process to produce the electrostatic image by the influence of heat rather than by light. This technique is sometimes referred to as electrothermography or thermoxerography and embodies an electrostatically chargeable layer of material which is sensitive to heat, rather than to light, to reduce its electrical resistance (increase its conductivity) to thereby cause the charge to leak away at the heat struck areas. The material is electrostatically charged and is exposed to radiant energy, e.g. infrared rays produced in a Thermofax machine, while under a pattern or master. The infrared rays are absorbed by the darker areas, e.g. the print, of the pattern and thereby preferentially heat these dark areas whereas they pass through the light ice areas, e.g. the non-printed areas, without heating them. The heat absorbed by the dark areas of the pattern is transmitted to the areas of the charged, heat sensitive, thermographic material opposite such dark areas to decrease the electrical resistance and increase the electrical conductance of the material at such areas and thereby cause the charge to leak away at these heat-struck areas. On the other hand, the non-heat struck areas of the material opposite the light areas of the pattern remain electrically charged so that an electrostatic image is formed on the material, from which a visible image is made by conventional xerographic toner development followed by conventional xerographic fixing or transfer.

Such proposals and attempts are described in US. Pats. Nos. 3,205,354, 3,132,963, 3,363,099, 3,128,198, 3,161,- 529 and 3,368,892.

Although these electrothermographic or thermoexerographic techniques have important advantages over the xerographic electrophotographic techniques particularly with respect to lower cost, none of them have achieved commercial success for a number of reasons. Resolution is not as good as with electrophotographic techniques; also image densities are not as uniform or as high and background densities are not as low.

These disadvantages of known electrothermographic techniques are in the most part overcome in accordance with the present invention by the use of a normally nonconduetive electrostatically chargeable material (i.e. material having a sufficiently high electrical resistance at normal temperatures to hold an electrostatic charge), which is not only sensitive to radiant electrothermographic energy or heat to change its electrical resistance (usually to lower such resistance and increase its conductivity to cause discharge of the electrostatic charge in the heat struck areas) and thereby provide a latent electrostatic image, but which is also at the same time activated by such energy or heat to change from a normal non-tacky state to a soft t-acky state at the heat struck areas and thereby provide a latent tacky, as well as a latent electrostatic, image. This provides better selective adherence of the toner particles to the latent image during toner development, thereby increasing density of the image and decreasing density of the background to provide a sharper image with better resolution and less background fog. At the same time the lower cost advantages of the electrothermographic technique are preserved.

The heat sensitive electrostatic chargeable material of the invention comprises a resin, preferably a thermoplastic resin, and a heat-activatable tackifying agent which is activated by the electrostatic image-forming radiant energy heat to convert the resin-tackifying agent composition at the heat struck areas from a normal substantially non-tacky solid state to a soft, tacky or sticky state which causes the toner particles to better adhere thereto. Actually the composition is converted by the heat to a highly viscous, slightly flowable semi-liquid. It is believed that the tacky state of the heat activated composition not only improves the final image by virtue of the fact that its tacky nature causes more toner particles to physically stick to it but also by virtue of the fact that it increases the voltage diflerential between the heat struck image areas and the non-heat struck background areas, as compared to conventional electrothermography, by increasing the conductance of the heat struck areas and by making the conductivity thereof more uniform.

i The heat-activatable tackifying agent is a plasticizerfor the resin. Accordingly, any known heat-activatable plasticizer for the particular resin involved can be used so long as it is heat-activatable by the heat conditions normally encountered in conventional electrothermography to render the resin-plasticizer system soft and tacky without making it soft and tacky under normal conditions.

Preferably the heat sensitive thermographic material of the invention is supported on and adhered to a conductive support or substrate, such aspaper, which affords a path for leakage of the charge from the heat struck areas and on which the material is coated.

The ratio'of tackifying agent to resin is substantially greater than one, preferably greater than 1.5 and more preferably greater than 2.0.

Preferably a colloidal stabilizer, such as a protein, is incorporated in the heat sensitive electrothermographic material as a coating levelling agent, i.e. to provide a more level uniform coating and to better adhere the coating to the support.

A number of proposals and attempts have been made to form a latent adhesive image in the duplicating art by the use of nonelectrostatic thermographic techniques, i.e. by exposing a heat sensitive duplicating material to radiant energy under a pattern to selectively heat the areas thereof under the dark areas of the pattern and thereby cause the material at such areas to become sticky so that the toner particles will adhere thereto. See US. Pat. No. 3,383,505, BritishPat. No. 1,091,488, Belgium Pat. No. 644,239 and Photographic Science and Engineering, vol. 10, No. l, January-February 1966. In one case, the heat sensitive material contains compounds from which the water of crystallization is driven off by the heat to wet the material and in another case the heat sensitive material contains compounds which are melted by the heat and exist in a supercooled wet condition. None of these proposals and attempts embody electrostatic imaging as in the case of the present invention. None of them have proven commercially successful for a number of reasons, e.g. resolution is poor, image density is low and background density is high.

DETAILED DESCRIPTION Example 1 Particulate poly-vinyl acetate (average particle size of less than two microns) and N-cyclohexyl-p-toluenesulfonamide (a solid plasticizer and heat-activatable tackifying agent for the polyvinyl acetate) sold by Monsanto under the name Santicizer 1-I-I were dispersed in water together with a-protein in an amount equal to 1% by weight of the dispersion. The ratio of sulfonamide to polyvinyl acetate was 4/ 1. The total solids content of the dispersion was 28% by weight. The mixture was mixed in a ball mill for four hours to fine grind the solids and get a good dispersion of the resin, plasticizer and a-protein particles in the water. I

The dispersion was applied to one surface of a conductive bond paper by a meier rod in an amount equal to 1.5 lbs. per ream of paper followed by drying. The resultant dry coating was 0.30 mil thick.

The coated surface of the resulting sheet was negatively charged to 210 volts with a conventional negative corona discharge unit.

The resulting charged sheet was then sandwiched against a mirror image copy of a master with the coated surface against the image side of the mirror image copy and the sandwich was subjected to infrared rays in a conventional Thermofax machine at a setting of 6 with the side of the mirror image copy opposite from the charged sheet facing the infrared lamp so that the infrared rays passed through the mirror image copy to the image thereon (direct imaging). The sandwich was separated and the resultant latent imaged electrostatic and tacky image) coated surface of the charged sheet was toner developed in conventional manner to give a visible image with a magnetic brush using a Xerox electrotropic toner (carbon black-thermoplastic resin) of negative polarity (triboelectric) containing iron filings as a carrier and brush.

The coating became tacky and discharged its electrostatic charge in the image areas due to infrared absorption upon exposure to the infrared lamp, whereas the background areas remained non-tacky and retained their negative charge to thereby form a tacky and electrostatic positive image. During development, the toner deposited quite densely in the image areas due to the tackiness as well as the reversal electrostatic development of the latent electrostatic and tacky image.

The toner developed sheet was then cleaned with fresh carrier, i.e. iron filings, to'clean the background areas followed by fixing the visual, cleaned image by heat in conventional manner to fuse the toner to the coating and thereby give a permanent image of excellent quality.

The imagewas a right reading imageof-the master. The resolution of the final image was excellent, with very high and uniform image density and low background density. The filling of the image area was excellent} When compared with an image made in the identical manner except that charging of the sheet was omitted to thereby provide a. tacky but non-electrostatic latent image, the resolution and sharpness of the Example 1 image was much better with higher 'ima'g'e density and lower background density. i I

When compared 'to an image made in the identical manner except that the sulfonamide was omitted to there'- by provide an electrostatic but not a tacky latent image, the sharpness and resolution of the Exhibit 1 image was much better, image density was higher and background density was lower.

Example 2 Example 2 was repeated except that (1) conductive transparent onion'skin paper. was, used instead of'bond paper and (2) exposure to infrared was a reflex exposure in which the uncoated surface of the coated sheet in the sandwich was facing the infrared lamp so that the infrared rays passed through such sheet. The results were comparable to Example 2.

Example 4 7 Example 2 was repeated except that (1) the master was used instead of the mirror image copy and (2) theuncoated surface of the coated sheet was held against the image surface of the master, so that the heat wastransmitted from the dark image areas of the master through the coated sheet to the coated surface thereof to image the .coated surface. The image on the coated surface was -.a

right reading image. The resolution and sharpness of the visible image was not nearly as good'asin Example 2. Also the image density was less an the background density was greater. 1

Example 5 Example 2 was repeated except that (1) the master was used in place of the mirror image copy and: (2) instead oftfixing the developed toner image, it was transferred 'to a transfer bond paper sheet by pressing the transfer sheet against the toner image and the transferred image was fixed. The image on the coated sheet wasqa mirror image of the master but thetransferredimage was a right reading image. The transferred image was lighter than the image of Example 2. I

Example 6 Example 2 was repeated except that (1) the master was used instead of the mirror image copy thereof and (2) a conductive transparent Mylar sheet was used instead of paper. The results were comparable to Example 2 and the image was right reading through the Mylar.

Example 7 Example 2 was repeated except that 1) the ratio of plasticizer to resin was 2 to l and (2) the Thermofax setting was 6. The sharpness, resolution, image density and background density of the resulting image was not as good as in Example 2. Also the latent tacky image was less tacky than in Example 2.

Examples 8 and 9 olution, image density and background density in both 7 cases were good but not quite as good as in Example 2.

Example 10 Example 2 was repeated except that an aluminum substrate was used in place of the paper. The results were satisfactory but not as good as in Example 2.

Example 11 Example 2 was repeated except that conductive gift wrap paper was used in place of bond paper. The results compared favorably with those of Example 2.

DESCRIPTION OF CLASSES OF MATERIALS, PROPORTIONS AND CONDITIONS Any thermoplastic resin can be used which has the following physical properties: (1) it must be an electrostatic chargeable thermoplastic resin which, by itself or when admixed with the plasticizer or tackifying agent, (a) has a relatively high electrical resistance, e.g. specific resistance of 10 ohm-cm. to 10 ohm-cm, and hence is nonconductive at normal temperatures to thereby take and hold an electrostatic charge at such temperatures, but (b) is sensitive to the elevated temperature and heat conditions encountered in conventional electrothermography, e.g., 100 F.250 F., to reduce its electrical resistance and become conductive and thereby cause discharge of the charged resin. Resins having these properties are well known in electrothermography; (2) it, itself or when admixed with the plasticizer or heat activatable tackifymg agent, must be non-tacky at normal temperatures but sensitive to such conventional electrothermographic elevated temperature and heat conditions to become softand-sl ght- 1y tacky or sticky. In a sense, the resin-plasticizer mix is a heat sensitive or heat activatable weak adhesive.

A primary function of the resin is a binder to bind the heat-activatable plasticizer or tackifying agent particles to each other when such agent is in the form of particulate solids, and to bond or adhere the resin-plasticizer system to the support sheet. It acts as a film former. Furthermore, the resin functions to control the tackiness achieved by the heat activatable tackifying agent at elevated temperatures, to reduce tackiness at normal temperatures and to contribute the desired electrostatic properties to the system. Without the resin, it is difiicult to form a cohesive layer which will adhere to the support sheet, and tackiness in response to heat may become excessive.

Resins which have the aforesaid properties and which may be used in the present invention include, in addition to polyvinyl acetate and styrene-butadiene rubber, other vinyl resins and polyesters such as polyvinyl chloride, after chlorinated polyvinyl chloride, copolymers of vinyl chloride and butadiene, copolymers of vinyl chloride and vinyl acetate, e.g. VYIHH, polystyrene, po'lyterephthalic acid ester, polyethylene, maleic acid resins, such as copolymers of styreneand maleic acid, copolymers of polyvinyl chloride and vinyl isobutyl ether (the polyethers), polyethylene, polypropylene, acrylonitrile, polyesters of isophthalic acid and ethylene glycol, natural rubber lattices, acrylic resins such as polymethacrylates, and poly-acrylates, waxes, etc. Hygroscopic resins which absorb moisture from the fingers or air, such as gelatin or polyvinyl alcohol, are not preferred.

Any known plasticizer for the particular resin can be used as a heat activatable tackifying agent so long as (1) when it is admixed with the resin, the system is dry and non-tacky at normal temperatures but is sensitive to electrothermographic heat to become soft and slightly tacky to thereby provide a tacky latent image and (2) when it is admixed with the resin, the system is non-conductive and hence electrostatically chargeable at normal temperatures but is sensitive to electrothermographic heat to become conductive to thereby provide an electrostatic latent image. Normally solid paticulate crystalline plasticizers are preferred in the form of discrete particles in the coating but normally liquid plasticizers may be used so long as when they are compounded with the resin, the system is dry and non-tacky.

Preferred plasticizers for use in the present invention are those which impart to the resin-plasticizer system a delayed tack after heating and cooling, e.g. 30 seconds to a minute or more.

Plasticizers or tackifying agents which are heat activatable under electrothermographic temperature conditions and which provide a delayed tack are known. See page 7 of Monsantos Technical Bulletin O/PL 1-H and US. Pats. Nos. 2,462,029, 2,608,542, 2,608,543, 2,613,191 and 2,613,156.

The delayed tack has the advantage that when the coated electrothermographic sheet and master (hereinafter the mirror image copy of a master and from which the images of the invention are made, as well as the master, will both be referred to as the master) are exposed to infrared followed by electrostatic charging, the latent tacky image is retained during charging and toning and during charging, toning and transfer when transfer techniques are used. Also this permits repetitive charging, toning and transfer or repetitive toning and transfer without repeating the exposure step.

The heat activatable tackifying agent should have a softening temperature, i.e. activation temperature, within the range of conventional thermographic temperature; created by infrared absorption, i.e. between F. and 250 F., more usually between 100 F. and 200 F.

Preferred tackifying agents are those which themselves are electrostatically chargeable at normal temperatures and are sensitive to heat to reduce the electrical resistance thereof, but this is not essential so long as the resin-plasticizer system has this property.

Suitable delay tack heat activatable tackifiers or plasticizers are the organic amide type plasticizers such as the sulfoanmides, particularly alkyl, aryl and alicyclic sulfonamides, e.g. N-cyclohexyl-p-toluene sulfonamide and N- ethyl p-toluene sulfonamide, and acetanilid, organic phosphate esters, particularly the triaryl, trialicyclic and tri (alkyl aryl) phosphates, such as triphenyl phosphate, tricresyl phosphate, dicyclohexyl phosphate and tri(p-tert.- butyl phenyl) phosphate, the phthalate esters, particularly the higher dialkyl and dialicyclic phthalates, such as dioctyl, dinonyl, didecyl, d-idodecyl phthalates and dicyclohexyl phthalate, and the terphenyls, such as o-terphenyl. All of these exhibit delayed tack characteristics when subjected to heat. The sulfonamides are best for the polyvinyl ester resins whereas the phosphates are best for the rubbers, e. g. natural rubber and butadiene styrene, and thephthalates are best for polystyrene.

The plasticizer may contribute to someextent to the cohesion of the coating and the adhesion thereof to the substrate.

Upon heating the resin-plasticizer system the change in physical condition of the coating to a tacky highly viscous liquid condition increases the conductivity of the heat struck areas to thereby provide an improved latent electrostatic image as well as a latent tacky image.

The heat-activatable tackifying agent should be one which renders the non-charged coating sufliciently 'tacky when exposed to conventional electrothermographic infrared heat conditions to provide good adhereneeof the noncharged toner particles to the heat struck areas but not so tacky that such" areas stick to the master against which the coating is held in contact duringinfrared exposure. The heat activated tackiness achieved. by any particular tackifying agent will vary depending on the particular resin used, the ratio thereof to the resin andthe intensity of the infraredsource. The tackifying agent should not render the system tacky under normal conditions, should not under normalconditions unduly decrease the electrical resistance of. the resin-tackifying agent system, and should not interfere with decrease in such resistance in response to electrothermographic heat. v a The heat activated tackiness of the resin-plasticizer system can be controlled by controlling the weight ratio of the tackifying agent or plasticizer to the resin. Increasing such ratio increases the tackiness achieved and decreasing it decreases the tackiness achieved since the tackiness is imparted to the system by the tackifying agent and not by the resin. Such ratio will depend on the particular tackifying agent and resin being used. Generally, such ratio should be greater than one to achieveadequate tackiness. A preferred weight .ratio is between 1.5. and 6,5; more preferred ratio being between 2 and 6. Excellent results have been achieved with a ratio of four although a ratio of three has also given satisfactory tackiness. As aforesaid, tackiness is in direct proportion to this ratio.

The minimum ratio is that at which minimum tackiness is achieved when the non-charged, resin-tackifying agent system is subjected to conventional electrothermographic heat. Minimum tackiness can be observed by toner developing the infrared exposed non-charged coating with non-charged tonerparticles. Any substantial adherence of the non-charged toner particles to the heat struckareas of the non-charged coating will provideadvantages to thereby provide a minimum tackiness. Optimum'tackiness and hence the optimum ratio is that at which the greatest amount of non-charged toner particles will adhere to the heat struck areas of the non-charged coating without such areas stickingto the master when they are in conventional electrothermographic contact with each other during exposure. The maximum tackiness and hence the maximum ratio is that beyond which the heat struck areas will stick to the master during exposure. Tackiness which results in sticking to the master will render'the latent and visible images indistinct because the coating materials run too much. Thus, the limits ofsuch ratio and the optimum ratio can be obtained by routine testing.

Plasticized resins have been suggested in' the "past for electrothermographic coatings but in all cases either the plasticizer was not one which is a heat activatable tackifying agent or the ratio of plasticizer to resin wastoo small to achieve a tacky latent image in addition to an electrostatic latent image. I

To a limited extent, tackiness can be controlled by.the setting of the Thermofax machine. Increased heatin most, but not all, cases provides increased tackiness. There is usually an optimum setting for any particular resin-tackifying agent combination, which can be observed by routine experiment.

The protein functions as a colloid stabilizer and acoating levelling and, bulking agent, i.e. it provides bulk and a more even, level coating layer on the support sheet. It

also promotes adhesion of the resin-plasticizer system to the base paper support.

Any protein can be used, such as dextran or starch. It can be omitted. When used, a preferred amount is between 0.57 and 2% or 3% by weight of the dispersion and between 0.11 and 0.8 or 1.2% by weight of the dried coating. The minimum amount is the minimum which will give the coating levelling effect desired. The maximum amount is determined by the fact that after optimum coating levelling is achieved, a greater amount contributes nothing to the electrothermographic properties of the coating and reduces the percentage-of the resin and tackifying agent in the dried coating which do contribute to suchproperties. Accordingly, the desired electrothermographic effect is reduced. It is preferred to use the minimum amount which will give an even, level coating.

If desired, a small amount of pigment, such as a white titanium oxide pigment, can be added to the coating in amounts up to 2 or 73% by weight of the coating.

The resin and plasticizer may be applied to the support base from an aqueous dispersion or emulsion as in the aforesaid examples or froma solution of, the resin and/or plasticizerin an organic solvent, such as toluene, alcohols and aromatic solvents, for oneor both, the non dissolved component when only one is dissolved being dispersed in the solution. When solutions are used, it is preferred to use a solution of the resin with the plasticizer dispersed therein. I

Where the resin and tackifying agent are applied as a dispersion of resin and plasticizer particles, such resin and plasticizer are present in the resulting dry coating in the form of discrete particles. When the coating is applied as a dispersion of plasticizer particles in a resin solution the resin in the drycoating is in the form of a continuous film having the plasticizer particles disbursed therein.

Where the resin and tackifying agent are applied as a dispersion, a preferred solids concentration is between 20 and 40% by weight,-a concentrationiof 25 to 35% by weight being more preferred. If the concentration is much less than 20% then removal of water .becomes a problem and the density of the dried coating is toov small.

If the solids concentration is much higher than 40%, the.

coating may be uneven and thick and will flake 01f and crumble. When solutions are used, the combined 60116611! trations of "the resin and plasticizer in the solvent is about the same as with a dispersion depending on the solubility characteristics.

An aqueous dispersion is preferred because it is more economical. i

The preferred dried coating weight is between 0.6 and 2.0 pounds per ream of paper. Optimum dry coating weight is between 1 to 1.75 pounds per ream of paper. It is advantageous to keep the coating as thin as possible since if it is too thick it may deleteriously afiect the feel of the paper, it increases cost and it may decrease resolution of the image and decrease adherence of the toner particles; The reason for this is that the larger mass requires more heat and the heat available with conventional thermographic techniques may notbe adequate.

Also where reflex imaging is used, the thicker the coating the more diflicult it is for the infrared to pass through it. Furthermore where the uncoated side of the duplicating sheet is applied to the master in order to obtain a right r'eading'image of the master, it is more difiicult for the heat absorbed by the dark areas to penetrate the duplicating sheet to the coating. v

'A preferred coating thickness is between 0.10 and 0.75 mil, more preferably between, 0.20 or 0.25 and 0.5 or 0.65 mil. InExamples 1 and 2, the thickness of the coating was 0.3 mil.

The support sheet should be one which "is conductive to provide a path for discharge of the heat struck areas.

-Any conventionalsupport sheet or substrate used in conas bond paper, gift wrapping paper and onionskin paper, other conductive cellulosic materials such as cellulose hydrate foils, cellulose acetate and cellulose acetobutyrate, conductive plastics (either themselves conductive or treated to make them conductive), such as polyesters, e.g. Mylar, polyamides, polyurethanes, polycarbonates, polystyrene, polyvinyl compounds, polyvinylrdene chloride, etc., metal foils or plates such as aluminum foil, and glass plates which have been made conductive.

When the coating surface is placed in contact with the source document to be duplicated during exposure, a transparent support such as Mylar, together with a transparent coating, should be used so the image can be viewed as a right reading image through the support and coating or else the mirror image produced must be transferred to a right reading image or else, as preferred, the source document with which the duplicating sheet is exposed should be a mirror image of the original master, which requires a preceding copy to be made from the original master, to thereby provide a right reading or non-reversed image. Otherwise, the coated side of the duplicating sheet should be away from the master, i.e. with the support against the master, in which case, the heat from the dark areas of the master must pass through the support and coating to the charged surface to provide a right reading image on the coated surface.

When paper is used, it is preferred to coat the smooth side of the paper.

The thickness of the support sheet is not particularly critical unless heat conductance through such sheet is being relied on, i.e. when the support surface is placed against the print of the master during exposure (in such case, the thickness of the support sheet should not exceed .005 inch). Also when reflex exposure is to be relied on, the support should not be so thick as to interfere with passage of infra red therethrough. Conventional support sheet thicknesses can be used. A preferred thickness is between 0.4 and mils. In Examples 1 and 2, a paper thickness of 2.5 mils was used.

Conventional thermographic machines can be used for infra red exposure, such as the 3M Thermofax machine.

Depending on the components of the coating, any setting over the range of settings of such machines can be used. For each coating, however, there is an optimum setting to provide optimum results.

Electrostatic positive or negative charging is carried out by conventional electrothermographic and electrophotographic xerography charging units and the applied voltage may vary over a wide range.

Any conventional developer toner used in xerography and electrothermography can be used and it is applied to the latent image in any conventional manner, such as by magnetic brush development, cascade development. cloud development, spray development, etc.

The toner is preferably in the form of solid toner particles, which are usually made by mixing a color pigment, such as carbon black, with a thermoplastic melt, e.g. polystyrene, and grinding the chilled mixture, which is used, as such, or with a carrier such as iron filings, glass beads, etc. However, conventional toners in liquid form can be used also. Xerox toner has been used successfully. Preferred toner particle sizes range from 1 to 5 microns average particle size. The larger the toner particle size the less the background. However, any conventional toner particle sizes can be used. The toner particles may be electrotropic, i.e. triboelectrically charged either negatively or positively. The carrier of the toner carries the charge and the colored toner particles stick or adhere to the carrier particles which act as a brush as well as a carrier. Where the coating has a negative charge, the use of negatively charged toner particles results in a positive image, i.e. the toner particles adhere to the heat struck discharged and tacky areas and are repelled by the negatively charged background areas.

The cleaning operation with the charged carrier with- 10 out toner particles, i.e. by brushing with the charged carrier, is conventional and conventional techniques can be used.

The toner is fixed in conventional manner by heat fusion, i.e. thermoplastic of the toner is fused to the coating, or chemically by solvent vapors. However, with the present invention, fixing may not be necessary in all cases because the tackiness of the heat struck coating holds the toner particles and when the coating loses its tackiness, the toner particles are securely bonded to the coating.

The coating is applied from solution or dispersion onto the support in conventional manner such as by coating rollers and meier rod.

If desired, both sides of the support sheet substrate can be coated with the resin-plasticizer coating and either or both surfaces electrostatically charged.

The invention has been described in detailed with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

We claim:

1. An electrothermographic duplicating sheet comprising a non-photoconductive layer of material adhered to a conductive base sheet, said material comprising an electrically chargeable resinous layer comprising a mixture of electrostatically chargeable resin and an organic heat-activatable tackifying agent which layer is activated by heat to convert it from a non-tacky state to a tacky state and to change the electrical resistance thereof, said layer forming both a latent tacky and a latent electrostatic image When both heated and charged.

2. An electrothermographic duplicating sheet comprising a layer of material adhered to a conductive base sheet, said material comprising an electrostatically chargeable resinous layer comprising a mixture of an electrostatically chargeable thermoplastic resin and an organic heat-activatable tackifying agent, which layer is sensitive to radiant thermographic heat applied to selective areas thereof to convert said selected areas from a normally substantially non-tacky state to a soft and tacky state to form a latent tacky image and to change the electrical resistance of said selected areas to form a latent electrostatic image.

3. A sheet according to claim 2, wherein said tackifying agent is a heat-activatable, delayed tack tackifying agent.

4. A sheet according to claim 2, wherein said resin is a thermoplastic resin and the ratio of tackifying agent to resin in said layer is substantially greater than one but less than about six.

5. A sheet according to claim 4, wherein said ratio is greater than 2.

6. A sheet according to claim 4, wherein said tackifying agent is a plasticizer selected from the group consisting of phosphate esters, phthalate esters, sulfonamides and terphenyls.

7. A sheet according to claim 4, wherein said resin is a resin selected from the group consisting of polyester resins, polyvinyl resins, natural rubber, synthetic rubber, polyurethane resins, polyether resins, and acrylic resins.

8. A sheet according to claim 4, wherein said layer also contains a protein as a levelling agent.

9. An electrothermogfiraphic duplicating sheet comprising a layer of material on a conductive base sheet, said material comprising an electrostatically chargeable resinous layer comprising a mixture of an electrostatically chargeable thermoplastic resin and an organic heat-activatable tackifying agent, which layer is activated by heat to convert said layer from a normally substantially non-tacky state to a soft and tacky state and to change the electrical resistance thereof, said layer being charged with an electrostatic charge and having a latent tacky and latent electrostatic image thereon.

11 12 10. A sheet according to claim 9, wherein said con- 3,519,463 7/1970 Baum 117-155 UA ductivc base sheet is a paper. 1 3,493,412 7/1970 Johnston et a1. 117155 UA References Cited ALFRED L. LEAVIIT, Primary Examiner UNITED ST P 1 5 M. F. Assistant Examiner 7 3,488,189 1/1970 Mayer 11 7 37 LE 3,519,464 2/1970 Balster et a1. 117155 UA 117--17.5, 37 LE, 155 U 

